Conventionally, active matrix liquid crystal display devices have been known as one of typical thin panel displays. In such liquid crystal display devices, as a display electrode substrate, an insulating substrate is used whereon switching elements such as picture elements, thin film transistors (hereinafter referred to as TFT) made from amorphous silicon, etc., are arranged in a matrix form.
The display electrode substrate is also arranged such that wirings including data signal lines, scanning signal lines, etc., for driving the picture elements are also formed on the insulating substrate so as to be connected to the TFT. As the insulating substrate, those of transmissive type, such as a glass plate, may be used.
The liquid crystal display devices of this type offer high quality display images. Moreover, the insulating substrate as a display electrode substrate does not have a strict restriction with respect to its area. Moreover, because of its adaptability to both the reflective type and the transmissive type, the practical applications of such liquid crystal display devices are enabled in a variety of fields.
The arrangement of an active matrix liquid crystal display device will be explained below. As shown in FIG. 12, in the liquid crystal display device, a plurality of data signal lines 42 and a plurality of scanning signal lines 43 are formed on an insulating substrate 1 in directions orthogonal to each other.
The data signal lines 42 and the scanning signal lines 43 three dimensionally cross with an insulating layer in-between, and a picture element 44 is formed so as to be surrounded by adjacent two data signal lines 42 and adjacent two scanning signal lines 43.
As shown in FIG. 13, an MOS-type transistor 45 that serves as a switching element is connected to a picture element 44. A source electrode of the transistor 45 is connected to the data signal line 42, a gate electrode of the transistor 45 is connected to a scanning signal line 43, and a drain electrode of the transistor 45 is connected to one of the electrodes of the picture element 44. The other electrode of the picture element 44 is connected to a common electrode as an earth electrode (not shown).
According to the described liquid crystal display device, in order to supply a data signal and a scanning signal to the picture element 44 provided with the transistor 45 (switching element), it is required to connect a driving IC as an external device. The driving IC is provided for driving the data signal lines and the scanning signal lines. The described connections can be made through the following two systems:
One of the systems is called "film-carrier system" wherein a signal is externally supplied from the driving IC through a connection film whereon a plurality of copper film signal lines are formed on a polyimide resin thin film base by making it in tight contact with a terminal array of the data scanning lines and a terminal array of the scanning signal lines formed on one of the substrates which constitute the liquid crystal display device. The other system is called the COG (Chip On Glass) system wherein the driving IC is directly mounted on a terminal formed on the peripheral portion of one of the substrates of the liquid crystal display device.
In order to improve the mounting efficiency of the driving IC, recently, the driver monolithic technique has been proposed wherein the driving IC (driver) is monolithically formed on the substrate in the manufacturing process of the substrate of the liquid crystal display device. According to the driver monolithic technique, since the number of input terminals of the liquid crystal display device can be reduced, a reduction in cost for mounting the components on a display module can be expected.
In the case of adopting the TFT composed of an amorphous silicon to the driving IC, because a sufficient driving power cannot be ensured, the arrangement is not suited for practical applications.
In order to counteract the described problem, it has been proposed that the TFT made of polycrystal silicon thin film is used as the switching element. The polycrystal silicon is used as a semiconductive layer formed on the insulating substrate. Such switching element composed of the polycrystal silicon thin film enables an improved drive capacity by more than ten times compared with the TFT made from amorphous silicon.
As a typical example of such TFTs made from the polycrystal silicon thin film, a polycrystal silicon thin film having a positive stagger structure will be explained in reference to FIG. 14. First, on the insulating substrate 41, a semiconductor layer 47 made from a polycrystal silicon is formed. After forming a gate insulating layer 48 and a gate electrode 49, a source electrode 50 and a drain electrode 51 are formed.
In the next stage, the insulating layer 52, metal wirings 53a and 53b and an insulating layer 54 are formed. Then, after forming a picture element electrode 55 composed of a transparent electrode (ITO: Indium-Tin Oxide), a protective film 69 is formed on an entire surface. Lastly, a light shielding metal film 70 is formed so as to cover the TFT composed of a semiconductive layer 47, a gate insulating layer 48, a gate electrode 49, a source electrode 50 and a drain electrode 51 from thereabove, thereby forming an active matrix substrate 56.
As a note, the light shielding metal layer 70 may be formed on a counter substrate 58 (to be described later). Although the above explanations have been given through the case of manufacturing the switching element in the picture element, the transistor which constitutes the driving IC may be manufactured in the same manner.
The manufacturing process of the liquid crystal display device including the substrate 56 will be explained next. A liquid crystal alignment film 59 made of polyimide, etc., is applied both on the substrate 56 formed in the semiconductor manufacturing process and on a counter substrate 58 having formed thereon a counter electrode 57. Thereafter, by rubbing a liquid crystal alignment film 59 with cloth, grooves which determine the orientation of the liquid crystal to be injected into the liquid crystal alignment films 59 are formed. In the described manufacturing process of the liquid crystal, high voltage of static electricity is likely to generate in the rubbing process.
In the next stage, two substrates 56 and 58 are bonded to one another while controlling the gap between them using a spacer, and a liquid crystal 60 is injected into the gap. Lastly, on both outer faces of the substrates 56 and 58, polarizing plates 61 are bonded, thereby obtaining a liquid crystal display panel.
In the described manufacturing process, static electricity may generate in the process of injecting the liquid crystal 60 and in the rubbing process of forming an alignment groove on the liquid crystal alignment film 59. Since the static electricity may cause an electrostatic breakdown in the picture element 44 of the driver monolithic liquid crystal display device and in the transistors which constitute the data signal line drive circuit and the scanning signal line drive circuit, the problem is presented in that the properties of the transistor vary.
As shown in FIG. 15, in order to solve the above-mentioned problems, a common wiring (short-circuit ring) is formed along the outer circumference of the insulating substrate 41 on the substrate 56 (display electrode substrate), and each of the input-output pads 63a and 64a of the data signal line drive circuit 63 and the scanning signal line drive circuit 64 and respective ends of the signal lines 42 and 43 develop short circuit.
By the described short circuit, even if a difference in potential occurs between the input-output pads 63a and 64a and between signal lines 42 and 43 due to the static electricity generated during the manufacturing process, the difference in potential is quickly dispersed to the input-output pads 63a and 64a and signal lines 42 and 43 through the common wiring 62, thereby preventing the problems that differences in potential are applied to the transistors as the switching elements. Therefore, the problem of lowering the property of the transistors and the insulation breakdown caused by the static electricity generated in the manufacturing process can be reduced.
However, in the described arrangement, since the signal lines 42 and 43 and input-output pads 63a and 64a develop short by the common wiring 62, it is difficult to perform defect checks of picture elements, disconnection checks of signal lines 42 and 43, and other defect checks.
In the method for manufacturing the liquid crystal display device, it is required, in its last stage, to make the signal lines 42 and 43, and input-output pads 63a and 64a electrically separated in order to drive each picture element 44, and to disconnect the common wiring 62 in the disconnecting process such as dicing process, laser beam emitting process, etching process, etc.
The described disconnecting process may damage the resulting substrate 56, and in order to perform the disconnecting process without damaging the substrate 56, a long time is required. Furthermore, the described static electricity proof structure is effective only in the manufacturing process, thereby presenting the problem that the protecting function against the surge input during the operation cannot be achieved by the described method.
The non-driver monolithic liquid crystal display device is disclosed, for example, in Japanese Laid-Open Patent Publication No. 49966/1993 (Tokukohei 5-49966). As shown in FIG. 16, the liquid crystal display device is arranged such that a protective circuit 65 is formed between at least one of the data signal line 42 and the scanning signal line 43 in the image display section and the common wiring 62 so as to be connected thereto.
In the protective circuit 65, two MOS type transistors 66 are connected in series, and the drain electrode and the gate electrode of the transistors 66 are respectively short-circuited.
In the described liquid crystal display device, when a normal operation voltage is applied to the data signal lines 42 and the scanning signal lines 43, one of the two transistors 66 connected in series is set in its ON position, while the other one of two transistors 66 is set in its OFF position. Therefore, in order to maintain the state where the signal lines 42 and 43 do not conduct, the operation of each transistor 45 as a switching element and each picture element 44 will not be disturbed. Therefore, disconnection checks of the signal wires 42 and 43 and defect check of the picture element 44 are enabled.
When an abnormal condition occurs, and a high voltage above the operation voltage is applied to the data signal line 42 and the scanning signal line 43, in the normal operation voltage, the transistor 66 set in its OFF position of the normal operation voltage is set ON by the breakdown between the source electrode and the drain electrode. Since the signal lines 42 and 43 are short-circuited, only a small potential difference develops between the signal lines 42 and 43, thereby preventing the electrostatic breakdown of the switching element.
In the described arrangement, when a voltage above the operation voltage, such as a static electricity, generates on the signal lines 42, or on the signal lines 43, the signal lines 42 and 43 are respectively short circuited to the common wiring 62 through the protective circuit 65 composed of two transistors 66, thereby reducing possible lowering of properties of the switching elements and the insulation breakdown due to the static electricity. This high voltage proof structure is effective also against the surge input in the normal operation.
According to the described conventional arrangement, when a normal operation voltage is applied, the signal lines and the pads are almost insulated. Therefore, disconnection checks and other defect checks are permitted. On the other hand, when a high voltage such as static electricity is applied, because of the breakdown of the transistor used in the protective circuit, the signal lines and the pads conduct so as to prevent possible damages and lowering of properties of the transistor against the applied high voltage.
As described, in the above-mentioned conventional arrangement, since the breakdown of the transistor, which is difficult to control, is involved, the problem is presented in that it is difficult to accurately set a turn-on voltage of the protective circuit.